init: add MALLOC_ARENA_MAX=1 to systemd #27641

This adds the MALLOC_ARENA_MAX=1 environment variable as suggested in https://github.com/bitcoin/bitcoin/blob/master/doc/reduce-memory.md#linux-specific to the systemd service file definition.

Without this env var, memory usage can grow significantly especially when rpcthreads is increased above its default value.

Closes #24542.

I have tested this change myself with positive results after dealing with memory consumption issues for a long time using systemd on a 8GB RAM raspi4. I figure a similar change may be desirable for the OpenRC and CentOS init files, but I don't have a way to test them and I'm not even exactly sure how environment variables should be added there (via export statements?).

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Looks like I goofed somehow when editing my branch to remove some whitespace and that resulted in closing this PR, that's what I get for creating the original commit in the github.com text editor. Reopened in #27642.